1. Field of the Invention:
The present invention relates to a fluid-filled bushing, and more particularly to a fluid-filled bushing for use in a joint, for example, between a radius rod and a vehicle frame of an automobile.
2. Description of the Relevant Art:
In some conventional automobiles, the automobile frame and a member which supports a rear wheel axle are connected by a radius rod through a rubber bushing for keeping the frame and the support member in a certain relative positional relationship. The spring constant (or rigidity) of the rubber bushing in the joint between the radius rod and the frame greatly affects the compliance of the automobile suspension.
Japanese Laid-Open Utility Model Publication No. 61-170736 published Oct. 23, 1986, filed by the assignee of the present application, discloses a fluid-filled bushing which can positively adjust the compliance of an automobile suspension to improve maneuvering stability of the automobile.
The earlier fluid-filled bushing comprises inner and outer tubes disposed coaxially with each other, and a rubber tube of large wall thickness interposed between the inner and outer tubes to join them. The rubber tube has an annular groove defined centrally in an outer peripheral surrface thereof fully therearound, the annular groove having a substantially channel-shaped cross section. The outer tube has a partition on its inner peripheral surface which is disposed centrally in the annular groove over its entire length to divides the space of the annular groove into two axially separated fluid chambers. The partition has an annular communication passage communicating with the fluid chambers and a hole defined radially inwardly and opening into the communication passage. A valve member is rotatably fitted in the hole in the partition for varying the cross-sectional area of the communication passage.
The outer tube is fixed to a vehicle frame whereas the inner tube is coupled to a radius rod. The valve member is actuated by a stepping motor or the like to vary the cross-sectional area of the communication passage through which a fluid flows for thereby controlling the compliance of the suspension.
In the above conventional fluid-filled bushing, however, the inner perpheral surface of the partition is held in sliding contact with the bottom of the groove of the rubber tube and is subject to large sliding resistance. Therefore, the range in which the suspension compliance is adjustable is narrow. Since the overall spring constant of the bushing cannot be determined unless the sliding resistance is taken into account, it has been tedius and time-consuming to design the bushing.